1. Field of the Invention
The present invention relates to a sound output device that outputs a masker (masking sound) for masking an operation sound generated by equipment.
2. Description of Related Art
Conventionally, to deal with an operation sound generated by an image forming apparatus or suchlike, a so-called “noise reduction technology” for reducing operation sounds is employed. Noise reduction lowers the sound pressure level of operation sounds, but does not completely solve problems of “harshness” and “discomfort” caused by operation sounds.
Aside from the noise reduction technology, there has also been proposed a sound masking technology as a technology to lower a loud sound (to a less perceptible level). Sound masking is a method taking advantage of a phenomenon (sound masking effect) in which perception of a sound at a certain level makes other sounds barely audible, and this method is mainly classified into frequency masking and temporal masking. More specifically, a loud sound is superimposed with a sound (masker or masking sound) mainly similar in frequency band to the loud sound, so that the loud sound is made barely audible, thereby reducing harshness and discomfort.
As a conventional sound output device applying the sound masking technology, there is a noise masking device described in Japanese Patent Laid-Open Publication No. 9-193506. This noise masking device is provided in an image forming apparatus or suchlike with a drive mechanism generating a loud sound (noise) during operation, and the noise masking device includes a sound generator for generating a masking sound to mask the loud sound, and a masking sound control unit for controlling the sound generator to generate a masking sound within a frequency range including the main component frequency of the loud sound. The masking sound control unit allows the masking sound to be generated within a frequency range between the lower and upper limits of a critical frequency band for the main component frequency of the loud sound.
In order for the noise masking device to further reduce harshness and discomfort, the sound pressure level of the masking sound may need to be raised. However, this results in a problem of increasing the power level of the sounds (the masking sound and the operation sound) emitted by the image forming device.